The person hiding in the mirror was Illuso from the Hitman Team. Fugo ends up being dragged into the mirror by Illuso's Stand, Mirrorman. Fugo then summons his own Stand, Purple Smoke. Despite Giorno wanting to find Fugo, Abbacchio decides to go look for the key, since that was their mission. However, once Abbacchio finds the key, he also gets dragged into the mirror world...
Carrying out this mission is 
the most important thing.
تنفيذ هذه المهمّة هو أهمّ شيء على الإطلاق
Die Mission zum Erfolg zu führen
hat oberste Priorität.
Llevar a cabo la misión
es lo más importante.
Llevar a cabo la misión
es lo más importante.
c’est ce qui m’importe le plus !
Portare a termine questa missione
è la cosa più importante.
Nada é mais importante
do que realizar esta tarefa!
унести отсюда ключ 
и доставить дочь к боссу!